Hazardous environments, such as chemical processing plants, fuel storage tanks, and the like, require extensive precautions to prevent accidental ignition of highly flammable mixtures of liquids, gases, and other material. For example, solenoid coils are often used to operate valves within these environments and the flammable material may accumulate within the coil enclosure. When that happens, a spark from a mechanical and/or electrical contact in the coil can ignite the flammable material, leading to potentially disastrous results for personnel and property. It is therefore important for solenoid coils used in hazardous environments to be able to confine or control any explosions that may occur within the coil enclosure to prevent such explosions from reaching the external environment and igniting the flammable material at large. As well, the solenoid coil and the electrical connections therein need to be protected from dust and debris that may be present within the environment.
Several safety measures exist for rendering a solenoid coil explosion proof. One safety measure involves making the enclosure or housing around the solenoid coil (and the electrical connections thereto) strong enough to contain any explosions occurring inside the enclosure. This means the housing must be able to withstand the pressure generated by such explosions without physically deforming and releasing the hot gases from the explosion into the exterior environment. The ability to withstand an explosion requires the housing to be quite thick and heavy and typically made of metal, although some non-metallic materials have been used. The housing must also be constructed in a manner to prevent any explosion occurring in the interior of the housing from propagating through seams and joints to the exterior environment. This means any seam or joint in the housing, such as from flanged or threaded joints, must comply with OSHA (Occupational Safety and Health Administration) or other industry “flame path” requirements to cool the explosion gases as they escape from the interior of the housing, thus preventing them from igniting any flammable material in the exterior atmosphere. Such “flame paths” require the use of specialized components as well as precision machining of the enclosure, which may add significant costs to the solenoid coil.
Another safety measure involves using a total and void-less encapsulation of the solenoid coil. Typically, a suitable encapsulation material having the proper electrical insulating and resistive properties is used to fill the space between the solenoid coil and the enclosure. Such an encapsulation-filled enclosure is sometimes called a “zero-volume” enclosure because the encapsulation leaves no room within the enclosure for flammable material to accumulate in proximity to the electrical connections. As there is no flammable material that can explode, a high strength and heavy enclosure is not required. A drawback of this approach is the encapsulation material used, usually a thermoset or thermoplastic material, tends to have less resistance to physical abuse and harsh and corrosive conditions and therefore may break down more quickly in many hazardous environments, thus compromising the integrity of the encapsulation.
Accordingly, a need exists for a solenoid coil for hazardous environments that provides improved protection from explosions and is more physically rugged at less weight and cost relative to existing solutions.